Semiconductor copper bond pad surface protection

ABSTRACT

Methods of protecting from atmospheric contaminants, or removing atmospheric contaminants from, the bonding surfaces of copper semiconductor bond pads by coating a bond pad with a layer of a ceramic material having a thickness that is suitable for soldering without fluxing and that is sufficiently frangible during ball or wedge wire bonding to obtain metal-to-metal contact between the bonding surfaces and the wires bonded thereto. Coated semiconductor wafers are also disclosed.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from U.S. Provisional PatentApplication No. 60/103,032 filed Aug. 27, 1998, and 60/127,249 filedMar. 31, 1999, the disclosures of both of which are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to methods for protectingsemiconductor copper bond pad surfaces with ceramic coatings that aresufficiently frangible during ball, wedge or flip chip bonding to obtainmetal-to-metal contact between the bonding surfaces and the wires bondedthereto. The method protects the copper bond pads during extendedexposure to water and water solutions such as are experienced duringsawing.

[0003] The use of copper bond pads on semiconductor devices would be anattractive alternative to that of aluminum, were it not for atmosphericcontamination of the copper surface, which oxidizes readily to form acoating that is not removable by standard methods of wire bondingmachines, and requires the use of fluxes in solder-type interconnects,e.g., flip chip bonding. Present attempts to overcome this probleminvolve the use of a cover gas that is unavoidably expensive and complexand restricts bond head and work holder movement, or the use of a noblemetal or overplating with inert metals which are more costly and canlead to the formation of unwanted intermetallic compounds at the bondpad interface.

[0004] U.S. Pat. No. 5,771,157 encapsulates a wedge bond of an aluminumwire to a copper pad with the resin, after the bond is formed. Noprotection against oxidation is provided to the copper pad prior towedge bonding.

[0005] U.S. Pat. No. 5,785,236 protects a copper bond pad from oxidationwith a surface layer of aluminum. This detracts from the advantagessought to be obtained by replacing aluminum bond pads with copper bondpads.

[0006] There remains a need for methods by which copper bond padsurfaces may be protected from oxidation prior to wire bonding or flipchip soldering.

SUMMARY OF THE INVENTION

[0007] This need is met by the present invention. Ceramic coatings havenow been developed for the bonding surfaces of copper bond pads that aresufficiently frangible to obtain metal-to-metal contact between thebonding surface and the wire bonded thereto during ball or wedge wirebonding, and to obtain a surface suitable for soldering without fluxing.

[0008] Therefore, according to one aspect of the present invention, amethod is provided for protecting from atmospheric contaminants, orremoving atmospheric contaminants from, the bonding surface of a coppersemiconductor bond pad by coating the bonding surface with a layer of aceramic material having a thickness that is suitable for solderingwithout fluxing and that provides the layer with a hardness that issufficiently frangible during ball or wedge wire bonding to obtainmetal-to-metal contact between the bonding surface and the wire bondedthereto.

[0009] The present invention thus provides semiconductor devices havingcopper bond pads with surface layer coatings that are capable of beingremoved at bonding or soldering. Therefore, according to another aspectof the present invention, a semiconductor device is provided containingat least one copper bond pad having a bonding surface coated with alayer of a ceramic material having a thickness that is suitable forsoldering without fluxing and which provides the layer with theaforementioned hardness.

[0010] This aspect of the present invention includes semiconductordevices having copper bond pads with surface layer coatings of rareearth metals that form complexes with copper. The surface layer has athickness that, upon formation of the copper complex and exposure to areducing environment containing hydrogen, forms a ceramic hydride layerhaving a thickness that is suitable for soldering without fluxing andwhich provides the layer with the aforementioned hardness.

[0011] This aspect of the present invention thus also includessemiconductor devices having copper bond pads with protective ceramicmetal hydride coatings. Therefore, according to another aspect of thepresent invention, a semiconductor device is provided containing atleast one copper bond pad having a bonding surface coated with a surfacelayer of a metal hydride compound selected from metal hydrides ofcopper-rare earth metal complexes and metal hydrides ofcopper-immiscible metals that form metal hydrides, in which the surfacelayer has a thickness that is suitable for soldering without fluxing andwhich provides the layer with the aforementioned hardness.

[0012] The inventive method provides the ability to bond wire to copperpads using existing equipment without modification of the wire bonder,and without additional costs and limitations of cover gas technology andhardware. The foregoing and other objects, features, and advantages ofthe present invention are more readily apparent from the detaileddescription of the preferred embodiments set forth below, taken inconjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

[0013] The sole drawing FIGURE is a schematic diagram of a methodaccording to the present invention, in which semiconductor devicesaccording to the present invention having copper bond pads with bondingsurfaces coated with hydride-forming materials and metal hydrides arealso depicted.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] The present invention forms protective ceramic coating layers oncopper bond pad bonding surfaces with thicknesses that are suitable forsoldering without fluxing. The ceramic layer thickness is selected toprovide at least the minimum hardness required for the layer to besufficiently frangible during ball or wedge wire bonding to obtainmetal-to-metal contact between each bonding surface and the wire bondedthereto.

[0015] Ceramic, rather than metallic, coatings are employed becausemetallic layers would be ductile and would plastically deform underimpact. Because ceramic materials cannot be deformed in the plasticregion, impact shatters the layer and allows it to be pushed asideduring wire bonding.

[0016] Essentially all commonly used ceramic materials have a hardnesssuitable for use with the present invention. One measure of ceramichardness is the Rockwell Superficial Hardness Scale (45-N) which isdefined in Somiya, Advanced Technical Ceramics (Prentice Hall, EnglewoodCliffs, N.J. 1996). Ceramic materials suitable for use with the presentinvention have a Rockwell Hardness (N-45) greater than about 38.

[0017] For purposes of the present invention, the meaning of the termceramic materials is adopted as it is defined in Callister, MaterialsScience and Engineering, An Introduction (3rd Ed., John Wiley & Sons,New York 1994) at page 4. Callister defines ceramic materials ascompounds between metallic and nonmetallic elements that are mostfrequently oxides, nitrides and carbides. Ceramic materials within thisclassification include ceramic materials composed of clay minerals,cement and glass. Ceramic materials are insulative to the passage ofelectricity and heat, and are more resistant to high temperatures andharsh environments than metals and polymers. Regarding mechanicalbehavior, ceramic materials are hard but very brittle.

[0018] A method and apparatus of the present invention are depicted inthe sole drawing FIGURE, in which bonding surface 12 of copper bond pad10 of a semiconductor device (not shown) is cleaned (Stage I). If thecopper surface is fresh and has not been exposed to a contaminatingatmosphere, Stage I cleaning is not required. In the depictedembodiment, the bonding surface 12 is coated with a layer 14 of ahydride-forming copper-immiscible metal or copper-complexing rare earthmetal (Stage II). For proper coating of surface layer 14, it isnecessary to reduce the oxides, hydroxides and sulfides that form on thesurface 12 of pad 10. Only after this reduction is complete can propersurface coating be performed. The surface 12 can be reduced by exposureto a reducing atmosphere, such as an atmosphere containing hydrogen, orby essentially any other conventional surface reducing techniques,including cleaning techniques such as plasma cleaning.

[0019] Examples of metals that are completely immiscible in copperinclude, but are not limited to, Ta, V and Nb. Examples of rare earthmetals that complex with copper include, but are not limited to, La, Y,and Ce.

[0020] The surface 12 of copper pad 10 is coated with metal layer 14 byconventional vapor deposition or analogous techniques. The rare earthmetals may require a heating step after deposition for the coppercomplex to form.

[0021] Surface layers of copper-immiscible hydride-forming metals can beformed by an alternative route. The copper-immiscible metal may beco-deposited with the copper as the copper bond pads are formed duringwafer fabrication. By heating the wafers after fabrication, theco-deposited immiscible metal will migrate to the surface of the copperbond pad, forming an oxidation-protective layer. Electroless orelectrodeposition techniques may also be employed.

[0022] The deposited layer 14 should be of a thickness capable offorming a frangible hydride layer. That is, the resulting ceramic layershould have a thickness sufficient to provide the layer with a RockwellHardness (N-45) greater than about 38. Suitable ceramic layers have athickness between about 10 and about 1,000 angstroms, with a thicknessbetween about 25 and about 500 angstroms being preferred.

[0023] When a rare earth metal is employed, it is preferably depositedin a layer thin enough to form an essentially pure copper complex. Thiscan be accomplished using rare earth metal layers with thickness fromabout 10 to about 1,000 Angstroms.

[0024] Copper-immiscible metal layers are preferably thin enough to becost competitive and permit ease of fabrication. For these purposes, thelayer 12 should be no thicker than {fraction (1/10)} the total combinedthickness of the pad 10 and layer 14. A thickness from about 10 to about1,000 Angstroms is preferred.

[0025] Layer 14 is then converted to a hydride layer (Stage III) byreduction with hydrogen, either by heating the bond pad in an atmospherecontaining hydrogen, or by exposing the bond pad to ahydrogen-containing plasma, e.g, plasma-cleaning operations. Onceformed, the hydride layer 16 is stable at room temperature.

[0026] It is not necessary for deposition or hydride conversion of thelayer 14 to be performed at the time of wafer fabrication. Bothprocesses can be done at a later time. As noted above, for properdeposition of the layer 14, the surface 12 of bond pad 10 must becleaned prior to deposition.

[0027] The hydride-formation step can take place at any stage prior towire bonding or flip chip bonding, so long as the reducing environmentis sufficiently aggressive enough to reduce the layer 14 to remove anyatmospheric contamination. Suitable reducing conditions can be readilydetermined by those of ordinary skill in the art without undueexperimentation.

[0028] The hydride layer 16 provides the surface 12 of bond pad 10 withoxidation resistance. Yet, because the hydride layer is frangible,conventional ball or wedge wire bonding can be performed to obtainmetal-to-metal contact between surface 12 and the wire bonded thereto(not shown), which also provides a surface prepared for solderingoperations.

[0029] The hydride compound rapidly disintegrates during wire bonding orsoldering by two mechanisms. One mechanism is mechanical, and derivesfrom the frangibility of the hydride layer. The hydride will alsothermally de-hydride during bonding, forming a hydrogen cover over thebond pad itself, which also prevents oxidation.

[0030] It is not necessary for the hydride process to be performed atthe time of wafer fabrication. The hydride process can take place at anystage prior to wire bonding or soldering, so long as thehydrogen-containing atmosphere is sufficiently aggressive enough toreduce any contaminants from the surface layer, and then subsequentlyhydride the surface layer.

[0031] The present invention also includes a single-step process inwhich the frangible ceramic coating is not a metal hydride. Instead, aclean copper bond pad is coated with a layer of a ceramic materialhaving a thickness that is suitable for soldering without fluxing andthat provides the layer with a Rcokwell Hardness (N-45) greater thanabout 38, so that the layer is sufficiently frangible during ball orwedge wire bonding to obtain metal-to-metal contact between each bondingsurface and the wire bonded thereto.

[0032] Examples of suitable ceramic materials include nitrides andcarbides of silicon, titanium and tantalium; oxides of aluminum,magnesium and zirconium; silicon and titanium dioxide; tungsten andboron carbide; and cubic boron nitride and diamond.

[0033] These coating layers are also formed by conventional vapordeposition or analogous techniques. The present invention thus providesbond pads with oxidation-resistant surfaces that can be ball or wedgewire bonded using conventional techniques without changes or additionsto current ball and wedge wire bonding or flip chip bonding processesand equipment.

[0034] The following non-limiting example set forth hereinbelowillustrates certain aspects of the invention, but is not meant in anyway to restrict the effective scope of the invention. All parts andpercentages are by weight unless otherwise noted, and all temperaturesare in degree Celsius.

EXAMPLE

[0035] Copper wafers having a copper thickness of at least 2,000angstroms were made via vapor deposition. Frangible ceramic coatings ofsilicon nitride with thicknesses between 10 and 1,000 angstroms wereformed via sputtering techniques.

[0036] Wire ball bonding was performed using various gold wires and aK&S Model 8020 wire bonder. The following wire bond process conditionswere employed:

[0037] Constant velocity=0.25-1.0 mil/msec.

[0038] Ultra sonic level=35-250 mAmp or equivalent power or voltagesetting

[0039] Bond time 5-50 msec.

[0040] Bond force 10-40 g

[0041] Free air ball diameter=1.4-3.0 mil

[0042] A variety of gold wire types were attempted and all were found tobe readily bondable: AFW-8, AFW-14, AFW-88, AFW-FP and AFW-FP2. Theharder wires, AFW-FP and AFW-FP2, performed the best.

[0043] A variety of bonding tools (capillaries) were used and all werefound to yield bondability in the bonded ball regions for which thecapillaries were designed. The best performing capillaries were partnumbers 414FA-2146-335 and 484FD-2053-335.

[0044] Copper wire was also bonded to the ceramic-coated bond pads. Aninert cover gas was employed for ball formation. The bond parameterswere not identical for those of gold for the same bonded ball size, butthe bond parameter range was not widely different for the range for goldball bonding onto copper substrates.

[0045] The foregoing description of the preferred embodiments should betaken as illustrating, rather than as limiting, the present invention asdefined by the claims. Numerous variations and combinations of thefeatures set forth above can be utilized without departing from thepresently-claimed invention. Such variations should not be regarded as adeparture from the spirit and scope of the invention, and are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method for protecting from atmosphericcontaminants, or removing oxidation from, the bonding surface of acopper semiconductor bond pad comprising coating a bond pad with a layerof a ceramic material having a thickness that is suitable for solderingwithout fluxing and that is sufficiently frangible during ball or wedgewire bonding to obtain metal-to-metal contact between the bondingsurface and the wire bonded thereto.
 2. The method of claim 1, whereinthe thickness of said coating layer provides said layer with a RockwellHardness (N-45) greater than about
 38. 3. The method of claim 1, whereinsaid coating layer has a thickness between about 10 and about 1,000angstroms.
 4. The method of claim 3, wherein said coating layer has athickness between about 25 and about 500 angstroms.
 5. The method ofclaim 1, wherein said ceramic material is selected from the groupconsisting of silicon nitride, silicon carbide, titanium nitride,tantalum nitride, aluminum oxide, magnesium oxide, silicon dioxide,titanium dioxide, zirconium oxide, tantalum carbide, tungsten carbide,titanium carbide, boron carbide, cubic boron nitride and diamond.
 6. Themethod of claim 1, wherein said coating step comprises the step ofexposing to a hydrogen-containing reducing environment a copper bond padhaving a bonding surface coated with a layer of a material selected fromthe group consisting of rare earth-copper complexes andcopper-immiscible metals that form metal hydride compounds.
 7. Themethod of claim 6, wherein said surface layer is formed immediatelybefore exposure of said bond pad to said reducing environment.
 8. Themethod of claim 6, wherein said step of exposing said bond pad to saidreducing environment comprises the step of heating said bond pad in areducing atmosphere comprising hydrogen or contacting said bond pad witha hydrogen-containing plasma.
 9. The method of claim 6, wherein saidsurface layer is coated on said bond pad by vapor deposition,electrodeposition or chemical deposition.
 10. The method of claim 1,wherein said surface layer is coated on said bond pad by vapordeposition.
 11. The method of claim 9, wherein said surface layercomprises a copper-rare earth metal complex formed by vapor deposition,electrodeposition or chemical deposition onto said copper bond padsurface of a rare earth metal that forms a copper complex.
 12. Themethod of claim 11, further comprising the step of heating saiddeposited rare earth metal surface layer to form said copper complex.13. The method of claim 11, wherein said surface layer consistsessentially of said copper-rare earth metal complex.
 14. The method ofclaim 6, wherein said surface layer comprises a copper-immiscible metal.15. The method of claim 14, wherein said copper-immiscible metal layeris formed by co-deposition of copper with said copper-immiscible metalto form said bond pad during wafer fabrication, followed by heating ofthe fabricated wafer so that said copper-immiscible metal migrates tothe surface of said bond pad to form said surface layer.
 16. The methodof claim 14, wherein said surface layer has a thickness substantiallyless than 20% of the combined total thickness of said bond pad and saidsurface layer.
 17. A semiconductor wafer comprising a device having atleast one bond pad that is coated with a layer of a ceramic materialhaving a thickness that is suitable for soldering without fluxing andthat is sufficiently frangible during bond or wedge wire bonding toobtain metal-to-metal contact between the bonding surface and the wirebonded thereto.
 18. The wafer of claim 17, wherein said ceramic materialis selected from the group consisting of hydrides of rare earth-coppercomplexes, hydrides of hydride-forming copper-immiscible metals, siliconnitride, silicon carbide, titanium nitride, tantalum nitride, aluminumoxide, magnesium oxide, silicon dioxide, titanium, dioxide, zirconiumoxide, tantalum carbide, tungsten carbide, titanium carbide, boroncarbide, cubic boron nitride and diamond.
 19. The wafer of claim 17,wherein said thickness of said layer of ceramic material provides saidlayer with a Rockwell Hardness (N-45) value greater than about
 38. 20.The wafer of claim 17, wherein said layer has a thickness between about10 and about 1,000 angstroms.
 21. The wafer of claim 20, wherein saidlayer has a thickness between about 25 and about 500 angstroms.
 22. Thewafer of claim 17, further comprising at least one wire that is ball orwedge bonded to said bond pad of said wafer device.
 23. The wafer ofclaim 17, wherein said device is a flip chip in which at least one wirelead is soldered to said metal hydride-coated bond pad.
 24. Asemiconductor wafer comprising a device having at least one copper bondpad having a bonding surface coated with a surface layer of a materialselected from the group consisting of copper-rare earth metal complexesand copper-immiscible metals that form metal hydride compounds, saidsurface layer having a thickness that, upon exposure to a reducingenvironment containing hydrogen, forms a hydride layer having athickness that is suitable for soldering without fluxing and thatprovides the layer with a hardness that is sufficiently frangible duringball or wedge wire bonding to obtain metal-to-metal contact between eachbonding surface and the wire bonded thereto.
 25. The wafer of claim 24,wherein said surface layer comprises a copper-immiscible metal.
 26. Thewafer of claim 25, wherein said surface layer is formed by co-depositionof said copper-immiscible metal and copper to form said bond pad duringwafer fabrication, followed by heating of said wafer so that saidcopper-immiscible metal migrates to said bond pad surface, therebyforming said surface layer.
 27. The wafer of claim 25, wherein saidsurface layer is formed by vapor deposition, electrodeposition orchemical deposition of said copper-immiscible metal onto said bondsurface.
 28. The wafer of claim 25, wherein said copper-immiscible metalis selected from the group consisting of Ta, V and Nb.
 29. The wafer ofclaim 24, wherein said surface layer consists essentially of acopper-rare earth metal complex.
 30. The wafer of claim 29, wherein saidcopper complex is formed by vapor deposition, electrodeposition orchemical deposition of said rare earth metal in a layer onto said bondpad surface.
 31. The wafer of claim 30, wherein said copper complexforms by heating said deposited rare earth metal layer.
 32. The wafer ofclaim 29, wherein said rare earth metal is selected from the groupconsisting of La, Y and Ce.
 33. A semiconductor wafer comprising adevice having at least one copper bond pad having a bonding surfacecoated with a surface layer of a rare earth metal that forms a coppercomplex, said surface layer having a thickness that, upon formation ofsaid copper complex and exposure to a reducing environment comprisinghydrogen, forms a hydride layer having a thickness that is suitable forsoldering without fluxing and that is sufficiently frangible during ballor wedge wire bonding to obtain metal-to-metal contact between eachbonding surface and the wire bonded thereto.
 34. The wafer of claim 33,wherein said rare-earth metal is selected from the group consisting ofLa, Y and Ce.
 35. The wafer of claim 33, wherein said surface layer isformed by vapor deposition, electrodeposition or chemical deposition ofsaid rare earth metal in a layer on said bond pad surface.
 36. Asemiconductor wafer comprising a device having at least one copper bondpad having a bonded surface coated with a surface layer of a metalhydride compound selected from the group consisting of metal hydrides ofcopper-rare earth metal complexes and metal hydrides ofcopper-immiscible metals that form metal hydrides, said surface layerhaving a thickness that is suitable for soldering without fluxing andthat is sufficiently frangible during ball or wedge wire bonding toobtain metal-to-metal contact between each bonding surface and the wirebonded thereto.
 37. The wafer of claim 36, wherein said surface layercomprises a hydride of a copper-immiscible metal.
 38. The wafer of claim36, wherein said copper-immiscible metal is selected from the groupconsisting of Ta, V and Nb.
 39. The wafer of claim 36, wherein saidsurface layer consists essentially of a hydride of a copper-rare earthmetal complex.
 40. The wafer of claim 36, wherein said rare earth metalis selected from the group consisting of La, Y and Ce.